1. Field of the Invention
The present invention relates generally to variable damping force shock absorber for an automotive suspension system. More specifically, the invention relates to a variable damping force shock absorber which can achieve linear damping force variation characteristics depending upon piston stroke speed with satisfactorily wide range of damping force variation.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-65930 discloses a variable damping force shock absorber for an automotive suspension system. The shown variable damping force shock absorber has a piston which has one axial end surface facing one of two fluid chambers. Three co-axial grooves are formed on the axial end surface. The grooves are closed by disc valve sealingly contacting with the outer surrounding edge of thereof. The disc valve is resiliently openable for establishing fluid communication between one of the fluid chambers and the other fluid chamber through respectively associated fluid communication paths for generating damping force. The disc valve is operable in a variable damping mode, in which when fluid communication is established only through the innermost groove and associated fluid flow paths, the shock absorber operates in HARD mode. On the other hand, when the fluid communication is established through the intermediate groove and the associated fluid paths, the shock absorber operates in MEDIUM mode. When the fluid communication is established through all grooves and all fluid paths, the mode of the shock absorber becomes SOFT.
In such conventional variable damping force shock absorbers, linear damping force variation characteristics can be achieved by multistage valve seats. However, on the other hand, since such conventional shock absorbers employ a sole disc valve, sufficiently high rigidity is required to obtain high damping force in the HARD mode. Such high rigidity of the disk valve may narrow damping force variation range in the SOFT mode. Therefore, setting of the valve disc stiffness becomes difficult.